In conventional hard disk drive systems, an integrated read/write head is used to write binary data to and read binary data from a recording medium. The recording medium, i.e., the disk, is coated with a ferromagnetic film that is either amorphous or contains tiny magnetic grains, each of which may be considered a magnet. For granular media, each magnetic mark is an area of the medium containing hundreds of grains. Data are recorded on the medium by causing the grains in each mark to align themselves so that their north-south poles point in the same direction. For this purpose, the write portion of the read/write head may induce a magnetic field in the region of the recording medium. The grains may be aligned, for example, longitudinally or perpendicularly relative to the plane of the disk. The read portion of the read/write head may use a magnetoresistive sensor.
Storage densities in hard disk drives using conventional longitudinal magnetic recording have been increasing at between 60% and 100% per year. In addition, storage densities in excess of 100 Gbit/in2 have been demonstrated. However, this trend may reach physical limits at densities beyond 1 Tbit/in2. One limiting factor is the so-called “superparamagnetic limit.”
The superparamagnetic limit may be described thusly. The need to maintain a given signal-to-noise (SNR) ratio requires that the size of the grains within a mark must be scaled with the area of the mark. However, as the size of the grains is reduced they become thermally unstable. In that connection, it has been determined that to be thermally stable for a period of ten years, the following relation must hold for granular media:                                                         K              u                        ⁢            V                                              k              B                        ⁢            T                          ≈                  40          -          70                                    (        1        )            where Ku is the media anisotropy energy density, V is the grain volume, kB is Boltzman's constant, and T is the temperature of the medium. Thus, as the volume of the grains is reduced, the anisotropy of the material must be increased. However, the coercive field Hc is proportional to the anisotropy and any increase in Ku results in an increase in Hc, ultimately making the medium unwriteable because it is not possible to generate arbitrarily large write fields with conventional magnetic heads. In the case of amorphous media, a similar limit exists.
One known technique of addressing this consideration is heat-assisted magnetic recording, in which a heat source, such as a focused optical beam from a laser, is used to reduce the coercivity of the medium during the writing process. This technique is used in standard magneto-optic recording. Heat-assisted magnetic recording allows for the use of high anisotropy media, which are quite stable at room temperature and which could not otherwise be written to with conventional thin film write heads.
Hybrid recording is a technique that combines heat-assisted magnetic recording for the write process, and sensitive magnetoresistive sensors for read back. For this type of recording, however, it is desirable to bring the optical field, the magnetic field and the read sensor of the integrated read/write head to the recording medium in a manner consistent with the storage density of the recording medium, which as described previously are headed for 1 Tbit/in2 and beyond.
For high-density storage applications, however, such as 1 Tbit/in2 and beyond, it is likely necessary to be able to produce intense optical spots as small as 50 nm or smaller, which is well below (such as by an order of magnitude) that which can be achieved with advanced diffraction-limited optical systems. Moreover, a further key enabling technology for future high-density hybrid recording systems is the capability to collocate the optical spot with the applied magnetic field.